In the manufacture of semiconductor devices, high purity materials are in great demand. The fabrication of semiconductor devices such as, for example, blue-light laser diodes presents a demand for ultra high purity magnesium metal. The development of double heterostructure blue-laser diode devices is particularly dependent on the quality of the material used for a cladding layer. One such material, a II-VI magnesium-containing compound ZnMgSSe grown by molecular beam epitaxy (MBE), is a highly effective material for cladding active p-type ZnSSe layers in the compound devices. This material can be lattice-matched to GaAs, and has a band-gap energy that is at least 0.3 eV higher than the active ZnSSe layer. Alternative magnesium semiconductor compounds, such as MgZnCdS, may be used to fabricate other color laser diodes.
Metallic impurities in such materials used for such cladding and other purposes are detrimental to the performance of semiconductor lasers. While high purity (99.9999%, or 6N[ine] pure) Zn, Se, S, and ZnS are employed in ZnMgSSe production, magnesium of only 4N pure (99.99% pure)is generally commercially available. Further, the highest purity of magnesium, considering all metallic impurities except zinc, that is currently available for research is between 99.9997% to 99.9998% metallic purity (high 5N pure). Such material is produced in a multi-step process by Dowa of Japan and is available at a price that is relatively high. But for color laser diode and many other semiconductor device applications, at least 6N purity magnesium is desirable.
The previously known methods for producing purified magnesium metals include the processes of zone refining, electro-refining, flux addition, precipitation, metallothermic reduction and distillation. The process of vacuum distillation, however, tends to produce the highest purity magnesium.
Purification of magnesium metal by vacuum distillation has been proposed, for example, by condensing magnesium vapor on a flat plate. Often, to achieve higher purities, magnesium must be distilled in two or more stages, at greater complexity and expense. It has been proposed to post-treat distilled magnesium with argon vapor, or to conduct a first distillation stage in the presence of argon and the second without argon. It has alternatively been proposed to purify magnesium in its molten state, for example by descending the liquid metal through a furnace or by melting the vapor deposited metal and purifying it in two or more stages.
Researchers have described purifying magnesium to greater than 99.999% purity with distillation columns formed of stainless steel and using graphite crucibles. Such process are claimed to produce 99.9995% to 99.9996% total metallic purity magnesium. Such columns have employed single-zone heaters and no temperature control over the distillation extent of the condenser. The total metallic purity, and the metallic purity excluding zinc content, of distilled magnesium has been reported at 99.9996% and 99.999905%, respectively, with zinc content being three ppm. These purities were attained, however, by using successive multiple distillations.
Researchers have also described employing columns with bubble-caps and shelf plates to purify magnesium, or the use of electromagnetic fields to increase purification capability. Magnesium alloys and magnesium sponge have also been described as purified by vacuum distillation. Other metals, alloys, and chemical compounds have been purified by vacuum distillation, including tellurium, neodymium, manganese, zinc, calcium, zirconium, titanium, aluminum alloy, silver iodide, and chlorides.
Other methods of purifying magnesium have been used including zone refining and electro-refining. Zone refining has been suggested using a horizontal graphite crucible. The electro-refining has been proposed using mixtures of chlorides, fluorides or oxides as electrolytes. The addition of flux, such as alkalis, alkali earth chlorides or fluorides, or boron or titanium halides, to molten magnesium has also been proposed to remove specific impurities. Impurity precipitation and thermal processing have been noted in literature, including metallothermic reduction and impurity oxide or halide reduction by silicon, zirconium, titanium or manganese.
Notwithstanding all of the efforts that have been expended to effectively and economically produce high purity materials such as magnesium at greater than 6N purity, the prior art has failed to produce magnesium and other such materials at such purity or to approach such purity from less than 4N purity material in a single step process at a reasonable price. Accordingly, there remains a continuing need for a method and apparatus for the effective and economical production of such high purity materials.